In general, the so-called “organic light emitting” phenomenon (organic electroluminescence) refers to a phenomenon in which electric energy is transformed into light energy by means of an organic substance. Particularly, when an organic film is disposed between an anode and a cathode and then electric potential is applied between both electrodes, holes and electrons are injected into the organic film from the anode and the cathode, respectively. When the holes and electrons injected as described above are recombined, excitons are formed. Further, when the exitons drop to a ground state, lights are emitted.
In addition to the above-described organic light emitting mechanism in which light emission is made by recombining of charges injected from both electrodes, there is another mechanism in which holes and electrons are not injected from external electrodes but are generated by an amphoteric charge-generating layer under the application of alternating current voltage, as in the case of a conventional inorganic thin film light emitting device, and the holes and electrons move to an organic thin film layer, resulting in light emission (Appl. Phys. Lett., 85(12), 2382-2384).
Since POPE, KALLMAN, et al. found electro-luminescence in anthracene single crystal in 1963, active research and development into OLEDs (Organic Light Emitting Devices) have been made up to now. Recently, organic light emitting devices have been used in flat panel display devices, lighting devices, etc. Such organic light emitting devices have been developed so rapidly that performance as display devices is remarkably improved and various applied products are developed.
In order to manufacture more efficient organic light emitting devices, many attempts have been made to manufacture an organic film in the device in the form of a multilayer structure instead of a monolayer structure. Most of currently used organic light emitting devices have a structure in which an organic film and electrodes are deposited. The organic film generally has a multilayer structure including a hole injection layer, hole transport layer, light emitting layer, electron transport layer and an electron injection layer.
It is known that OLEDs are characterized by high brightness, high efficiency, low drive voltage, color changeability, low cost, etc. However, in order to have such characteristics, each layer forming an organic film in a device (for example, a hole injection layer, hole transport layer, light emitting layer, electron transport layer and electron injection layer) must be formed of more stable and efficient materials.
A method of doping a light emitting host with a fluorescent compound so as to increase the light emitting efficiency of a multilayer-structured OLED was disclosed. Particularly, according to Tang, et al. (J. Appl. Phys. Vol. 65 (1989), p. 3610), light emitting efficiency can be improved by mixing a fluorescent compound having a high quantum efficiency (for example, coumarin pigments or pyran derivatives) in a small amount with a light emitting host. In this case, light having a desired wavelength can be obtained depending on the type of the fluorescent compound. However, when Alq3 is used as electron transport material and drive voltage is increased to obtain high brightness, green light emission based on Alq3 may be observed in addition to light emission based on the doped fluorescent compound. This is problematic in terms of color purity, particularly when the color of light to be emitted is blue. It is known that such a problem results from a narrow band gap between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of Alq3. Such a narrow band gap results in exciton diffusion from a light emitting layer to Alq3, thereby causing light emission based on Alq3.
The use of hole block material has been reported as another method for increasing light emitting efficiency of OLEDs, wherein the hole block material includes 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), bathocuproine (BCP), etc. (Jpn. J. App. Phys. Part 2, 1993, 32, L917). However, the above-mentioned materials show poor durability and have a serious problem of deterioration of a device, particularly when the device is subjected to continuous light emission while being stored at high temperature. Moreover, there are additional problems in that the above-mentioned materials should be provided as a layer separated from a light emitting layer, and that drive voltage increases due to a large band gap between the HOMO and the LUMO when the materials are used.
Therefore, in order to overcome the problems occurring in the prior art and to further improve characteristics of OLEDs, it is necessary to develop more stable and efficient materials that may be used in OLEDs.